FastJarFS was behaving differently than the regular CoreJarFS it
tries to replace, namely it was cleaned after each compilation
(had the lifetime tied to core env).
This commit preserve it in the app env the same way as CoreJarFS,
so it could be reused in parallel builds and preserved if automatic
cleanup of the app env is turned off,
But since FastJarFS caches also mmf handles, the additional handles
cleaning is introduced that triggers after all possibly parallel
compilations are finished.
Ilya Chernikov
7d5257c258